you can't import an avi or any other file except the apple formats into itunes.

you need to install perian and then create a reference file to the actual video using quicktime pro. the reference file will be a few kilobytes, and you can import that into itunes. when itunes plays the file, the reference file tells it where the avi file is and starts playing it. a bit convoluted but works well, especially if you want to use frontrow + itunes instead of just frontrow + movies folder.

Perian is basically all the codecs you need, wrapped into one install. You don't really need DIVX installed once perians installed.Chances are you'll find the files that didn't have sound before, do now thanks to Perian. Chances are they're AC3 (Dolby Digital).If you still can't get it to play properly, you should also have VLC installed. VLC generally works 99% of the time, but I prefer Quicktimes interface - if its working in quicktime it will play on anything else that uses quicktime - FCP, Frontrow, etc....

You could delete DIVX, but as you're new to this, and chances are things will still play fine with both installed. Just leave it.

Perian 1.1 is the newest version, you'll get a message when a new comes out, although there hasn't been much activity as 1.1 is pretty solid.

VLC = Video Lan Client. It's based on mPlayer, the Linux playback engine for media, it plays pretty much everything you can throw it. It's multiplatform so runs on Windows, Mac, linux. The only downside is that its interface, though mostly complete, just isn't a solid as quicktime.

Frontrow = Press Command + Esc on you Mac and see a pretty interface for accessing you media. Best with a remote and maybe a Big Screen connected.